『Abstract
The mobility of titanium, zirconium and thorium, elements commonly
considered insoluble during supergene weathering, is still not
well understood, especially in intensely weathered regolith. Thus,
an intensely weathered lateritic profile (JG) developed on meta-granitoids
in Jarrahdale, Western Australia, was investigated. The mobility
of Ti, Zr and Rh has been assessed at both mineral assemblage
and profile scales and the mode of occurrence has been investigated
through the combined use of geochemical data from bulk regolith,
particle size fractions and sequential extractions, with in situ
data determined by electron probe micro-analyzer and synchrotron
X-ray powder diffraction. Neoformed poorly crystalline phases
containing trace to minor amounts of Zr, Ce and Th unassociated
with silicates or phosphates were identified on the walls of Al/Fe-rich
pores in the ferruginous duricrust. This implies that some mobilization
and redistribution of Zr and Th occurs within a sample scale.
Breakdown of primary thorite and rare earth element rich fluorocarbonates
is thought to be the source for Zr and Th in the neoformed phases
rather than zircon. Thus, the mineral hosts of Zr, Ti and Th in
the parent rock and their relative susceptibility to weathering
are the fundamental controls on subsequent mobility during initial
weathering. Trace amounts of Th in secondary phases, such as rhabdophane
and florencite, show translocation of Th at the mineral scale,
while strong partitioning of Th into gravel rather than matrix
reflects redistribution of Th at the profile scale. The absence
of primary sphene from the regolith and the presence of partially
dissolved ilmenite and rutile grains in the ferruginous mottled
zone suggest mobilization and translocation of Ti at a mineral
assemblage scale. Furthermore, the fluctuation of Ti/Zr in the
ferruginous zone is in contrast to the consistency of Zr/Hf throughout
the profile in general (within the range of parent meta-granitoid).
This suggests that Ti and Zr fractionate from each other and partition
between gravel and matrix during extreme weathering and advanced
lateritization. This study demonstrates that Ti, Zr and Th are
mobile at a variety of scales, an important consideration that
is often overlooked when calculating element mass flux in intensively
weathered regolith.
Keywords: Zirconium; Titanium; Thorium; Laterite; Weathering;
Regolith』
1. Introduction
2. Materials and methods
2.1. Description of the study areas and sampling
2.2. Analytical methods
2.3. Mass balance calculation
3. Results
3.1. Bulk titanium, zirconium and thorium concentrations
in regolith
3.1.1. Abundance of titanium, zirconium and thorium in the parent
rock and regolith
3.1.2. Variation of ratios of titanium, zirconium and thorium
with depth
3.1.3. Mass balance of titanium and thorium
3.2. Mineralogical characteristics of titanium, zirconium and
thorium in the JG profile
3.2.1. Occurrence of titanium, zirconium and thorium in parent
meta-granitoids
3.2.2. Occurrence of titanium, zirconium and thorium in the
lateritic regolith
3.3. Spot analyses of the iron nodules
3.4. Grain size distribution of titanium, zirconium and thorium
in the lateritic regolith
3.5. Partitioning of zirconium and thorium into different extraction
species
4. Discussion
4.1. Mode of occurrence of zirconium and thorium in the lateritic
regolith
4.2. Sources of zirconium in poorly crystalline phases in ferruginous
duricrust
4.3. Partitioning of thorium between gravel and matrix
4.4. Mobility of titanium in the JG profile
4.5. Geological parent mineralogy vs. weathering conditions
5. Conclusions
Acknowledgments
Appendix 1. EPMA detection limits for element analyses in minerals
(Figs. 5 and 6) from the parent meta-granitoid and lateritic regolith
of the JG profile
Appendix 2. EPMA detection limits of element concentrations in
Table 5 in iron nodules from the upper ferruginous zone of the
JD profile
References